Method and circuit for interconnecting a plurality of inductors at the output transformer secondary

ABSTRACT

A method and circuit for interconnecting a plurality of inductors relative to each other in order to reduce the reflected impedance at the secondary portion of an output transformer. The method comprises separating the inductors into separate groups with each group having the same number of inductors therein; electrically interconnecting the inductors of each group in series; and, electrically interconnecting the groups in parallel to each other for electrical interconnection to the output transformer secondary.

United States Patent [191 Killian [451 June 5, 1973 54] METHOD AND CIRCUIT FOR 2,828,397 3/1958 Emerson ..219 10.43 INTERCONNECTING A PLURALITY OF 2,858,405 10/1958 Kimbrough et al .219] 10.75 INDUCTORS AT THE OUTPUT, 2,714,648 8/1955 Blok et al. ..219 10.79 TRANSFORMER SECONDARY 3,477,657 2/1970 Lavins ..219/10.67

[75] Inventor: William H. Killian, Cuyahoga, Ohio Primary Examiner Thomas Kozma [73] Assignee: Park-Ohio Industries, Inc, Cleve- {ii-$ 99?! Reynolds la d, Ohi Attorney-James H. Tilberry, Alfred C. Body and 22 Filed: Feb. 28, 1972 Robert Vlckers 21 Appl. No.: 229,894 [57] A R CT A method and circuit for interconnecting a plurality of [52] "219/1035, 219/ 219/1079 inductors relative to each other in order to reduce the [5;] "I ..H05b 5/06 reflected impedance at the Secondary portion of an [5 1 e 0 35 36 output transformer. The method comprises separating the inductors into separate groups with each group [56] References Cited having the same number of inductors therein; electrically interconnecting the inductors of each group in UNITED STATES PATENTS series; and, electrically interconnecting the groups in parallel to each other for electrical interconnection to Denner ..219/10.75 X the output transformer secondary.

McBrien .;.....219/l0.43 X

3 Claims, 3 Drawing Figures METHOD AND CIRCUIT FOR INTERCONNECTING A PLURALITY OF INDUCTORS AT THE OUTPUT TRANSFORMER SECONDARY This application? pertains to the art of induction heating and more particularly to simultaneous induction heating of a workpiece at a plurality of separate locations.

The invention' is particularly applicable to a method and circuit for interconnecting a plurality of inductors at the output transformer secondary when a workpiece such as, for example, an automobile engine block is to have the exhaust valve seat portions simultaneously heated and quenched and will be described with particular reference thereto; however, it will be appreciated that the invention has broader applications and may be used in other instances where it is desired to simultaneously inductively heat a plurality of like areas on a workpiece. The commonly assigned patent application Ser. No. 151,493 filed June 9, 1971 is incorporated hereinto by reference to merely show an arrangement wherein the subject invention may be employed.

With the advent of low level gasoline, it has become necessary to harden the exhaust valve seats of internal combustion engines to prevent premature wear. To effect this operation, a plurality of induction heating inductors are positioned adjacent to the several exhaust seats in the head of an internal combustion engine. These inductors are then simultaneously energized by a single transformer secondary to heat inductively the individual valve seats which are subsequently quench hardened. In this type of arrangement, the separate inductors were connected in electrical series with each other to the secondary circuit of a high frequency transformer. In most instances, the output transformer itself was of the radio frequency type and was comprised of a multi-turn primary portion surrounded by, and in close spaced relationship therewith, a thin, sheet-like secondary portion, this type of transformer is most often used when frequencies in the range of 15 kc and above are desired to effect heating and its use is known in the art. In this type of prior arrangement,

' the load, that is, the workpiece areas being heated by the plurality of inductors reflect an impedance back to the secondary portion of the transformer itself. This reflected impedance, since the inductors are interconnected in electrical series, is generally equal to the sum of the individual impedance values at each inductor. Such induction heating apparatus employ oscillators which generally include an oscillating tank circuit comprised of at least one capacitor that is tuned to the inductive reactance of the tank circuit. This inductive reactance includes the impedance reflected from the in-.

ductors. In such a power supply, it is desirable to maintain a low reflected resistance in the tank circuit. The impedance reflected back to the secondary from the inductors includes high resistance and is reflected back through the transformer primary into the oscillating The present invention contemplates a new method which overcomes all of the above referred problems and others and provides a method for reducing the reflected impedance into the secondary portion of a high frequency output transformer which method is simple, economical to employ, variable to accommodate different types of workpieces and provides for more efficient operation of the circuit. v

In accordance with the present invention, there is provided a method for reducing the reflected impedance into the secondary portion of a high frequency output transformer connected to an oscillator wherein the secondary portion is connected so as to simultaneously supply high frequency electrical energy to a plurality of induction heating inductors. The method comprises the steps of:

a. dividing the plurality of inductors into a plurality of inductor groups;

b. electrically interconnecting the inductors of each of the groups in series relationship with each other; and,

c. electrically interconnecting the groups in parallel relationship with each other for interconnection to the secondary portion of the output transformer.

In accordance with a limited aspect of the present invention, the method further includes the step of providing an equal number of the induction heating inductors in each of the inductor groups.

In accordance with another aspect of the present invention, there is provided a circuit which permits controlling the reflected impedance into the secondary circuit portion of a high frequency output transfomier connected to an oscillator wherein the transformer is employed to simultaneously supply high frequency electrical energy to a plurality of induction heating inductors.

The principal object of the present invention is the provision of a method and circuit for reducing the reflected impedance into the secondary portion of a high frequency output transformer.

Another object of the present invention is the provision of a method and circuit for reducing the reflected impedance into the secondary portion so as to increase the operational efficiency'of an oscillating tank circuit connected therewith.

Still another object of the present invention is the provision of a method and circuit for reducing the reflected impedance into the secondary portion of a high frequency output transformer and which permits varied circuit configurations so as to accommodate varied workpiece designs and positionings.

The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein:

FIG. 1 is a cross sectional view of a radio frequency transformer and showing schematically the circuit employed therewith to achieve the concepts of the subject invention;

FIG. 2 is a plan view of the radio frequency transformer shown in FIG. 1; and,

FIG. 3 is a schematic showing an alternate circuit which may be employed in practicing the concepts of the subject invention.

Referring now to the drawings wherein the showings are for purposes of illustrating the preferred embodiment of the invention only and not for purposes of limiting same, the FIGURES show a radio frequency transformer A having high frequency electrical energy supplied thereto from a high frequency electrical energy supply area B and an induction heating zone C.

More specifically, transformer A includes a primary portion comprised of a hollow, multi-turn copper coil having a generally cylindrical overall shape with a first lead 14 at one end thereof and a second lead 16 at the other end thereof. As coil 12 is hollow, it permits the passing of a cooling fluid such as water therethrough from a cooling fluid inlet 18 adjacent lead 14 and out through a cooling fluid outlet 20 adjacent lead 16. The secondary portion generally designated 28 of the transformer is formed from a thin sheet-like member 30 of, for example, copper. This member is generally coaxial with and substantially surrounds coil 12 so that there is a small air gap A between the outermost lateral portions of the coil and the surface of the sheetlike member. The thin sheet-like member includes two longitudinally extending closely spaced edges 32, 34 each connected to electrical leads as will hereinafter be more fully described. Disposed to conveniently extend around the outside of member 30 is a hollow cooling coil 36 having a cooling fluid inlet 38 and cooling fluid outlet 40. Cooling coil 36 is affixed to the outside of member 30 by any convenient means such as, for example, welding or brazing. During operation of the device, a cooling fluid such as water is passed through coils 12, 36 so as to retain the primary and secondary portions of the transformer at a temperature below their melting temperatures. It should be noted that the construction of radio frequency transformer A is generally conventional and is of the type of transformer generally employed when frequencies of above about l5 kc are desired to effect induction heating.

High frequency electrical energy supply area B includes lead lines '50, 52 extending from a high frequency oscillator (not shown) to leads 14, 16 respectively. Line 50 includes a capacitor 54 and coil 56 therein and a capacitor 58 is disposed between lines 50,

52 in parallel relationship therewith. Line 52 is further connected to ground 60. It should be here noted that the high frequency electrical energy supply area B is also deemed conventional and known in the art.

Induction heating zone C includes a plurality of individual inductors generally designated 70 and, in the preferred embodiment of the invention, comprise block inductors particularly designed and positioned for inductively heating the exhaust valve seat areas of an automobile engine block. It will be noted that in the preferred embodiment shown, six of the inductors 70 are shown. The specific design of the inductors themselves does not form a part of the present invention and are therefore not more fully shown in the drawings or described herein. The commonly assigned patent application having Ser. No. 151,493, filed June 9, 1971 is incorporated herein byreference as merely showing a type of operation wherein the subject invention may be advantageously employed. The inductors themselves are interconnected by a line 72 and are divided into two groups generally designated 74, 76, with three inductors in each group. Groups 74, 76 are connected in parallel with each other and to secondary portion 28 by lines 78, 80 at edges 32, 34 respectively. Because of this type of parallel arrangement between an equal number of the inductotal resistance found therein and the total resistance of a parallel circuit is equal to the product of the total resistance of each leg divided by the sum of the resistance in each leg. Thus, the above described parallel arrangement substantially reduces the reflected impedance or resistance into secondary portion 28 of output transformer A in order that the resistance created by operation of inductors themselves will be reflected back to the oscillating tank circuit in a lesser amount in order that a more efficient operation may be realized within the tank circuit.

A second embodiment of the present invention is shown in the circuit and induction heating zone C of FIG. 3. In that FIGURE, like components have like numerals and new components have new numerals. As shown therein, each inductor 70 is electrically connected to the secondary portion of transformer circuit A by yet another transformer circuit. In FIG. 3, these other transformer circuits are shown as having a primary portion and secondary portion 92. Secondary portions 92 are fixedly positioned relative to their associated inductors 70 and are movable relative to primary in directions a, b by convenient force means generally shown by numeral 94. The force means may comprise, for example, pneumatic or hydraulic means, the specifics of which do not form a part of the present invention and are, therefore, not more specifically shown and described. The individual movement of inductors 70 permits precise location thereof relative to the exhaust valve seat which is to be inductively heated thereby. Although there are differences in structure in the embodiment shown in FIG. 3 and described hereinabove, the operation and desirable characteristics achieved thereby are the same as those described with reference to FIG. 1.

Although induction heating zone C shows six inductors 70 with three in each leg of the parallel circuit, it will be appreciated that other arrangements may be employed using additional groups similar to groups 74, 76 or utilizing an additional number of inductors 70 to accommodate varying workpiece heating requirements. In using the subject invention, it is merely necessary that each group include the same number of inductors.

The invention has been described with reference to the preferred embodiment. Obviously, the modifications and alterations will occur to others upon the reading and understanding of this specification. It is my intention to include all such modification and alteration insofar as they come within the scope of the appended claims or the equivalents thereof.

Having thus described'my invention, I now claim:

1. A method of controlling the reflected impedance in the secondary circuit portion of a high frequency output transformer connected to an oscillator wherein said secondary circuit portion is connected so as to simultaneously supply high frequency electrical energy.

to an even number of windings, each of said windings forming the primary of a transformer having a secondary winding connected to an inductor, said method comprising the steps of:

dividing said windings into two winding groups, each having an equal number of windings;

electrically coupling the windings in each of said groups in a series relationship relative to each other; and,

electrically coupling said two groups of windings in a parallel relationship relative to each other.

2. A circuit for controlling the reflected impedance.

into the secondary circuit portion of a high frequency output transformer connected to an oscillator wherein said secondary circuit portion is connected so as to simultaneously supply high frequency electrical energy to a plurality of induction heating inductors, said circuit comprising:

transformer for electrically coupling said plurality of ductors. 

1. A method of controlling the reflected impedance in the secondary circuit portion of a high frequency output transformer connected to an oscillator wherein said secondary circuit portion is connected so as to simultaneously supply high frequency electrical energy to an even number of windings, each of said windings forming the primary of a transformer having a secondary winding connected to an inductor, said method comprising the steps of: dividing said windings into two winding groups, each having an equal number of windings; electrically coupling the windings in each of said groups in a series relationship relative to each other; and, electrically coupling said two groups of windings in a parallel relationship relative to each other.
 2. A circuit for controlling the reflected impedance into the secondary circuit portion of a high frequency output transformer connected to an oscillator wherein said secondary circuit portion is connected so as to simultaneously supply high frequency electrical energy to a plurality of induction heating inductors, said circuit comprising: transformer for electrically coupling said plurality of inductors to said secondary circuit portion; first means for connecting said transformer means into a circuit; means for dividing said series circuit into two branches, each containing the same number of said series connected transformer means; and, second means for connecting said two branches into a parallel type electrical relationship relative to each other.
 3. The circuit as defined in claim 2 wherein said transformer means each comprises a transformer having a secondary winding connected to one of said inductors. 